Yes, I agree - they had a lot of clever, funny, well-observed and well-executed ideas that were spoiled by the unpleasant ones.
I loved the idea of the budget Irish airline Our Lady Air flying people to Barcelona, but to a cheaper airport a bit further out from the centre, to save on costs... and that airport 'just outside' Barcelona happened to be in Ireland with a 2,000+ mile shuttle bus into the city centre!
Arguably, they could have kept the Precious character as she was, but had her played by somebody like Andi Osho, Gina Yashere or Jocelyn Jee Esien... but they prided themselves on playing all of the characters themselves, so I'm sure they wouldn't have wanted that.
I suppose I can sort of see the basic thinking in being very versatile as an actor as an impressive thing; it's kind of the whole point of acting, unless you're somebody like Hugh Grant and only ever play posh white male Hugh Grant-type characters... but there are socially-accepted boundaries, which blacking up very definitely crosses over by a mile.